1. Field of the Invention
The present invention relates to a TM mode dielectric resonator having a frame and a dielectric resonant element, wherein the frame has concave portions formed from the external surface of the frame toward the dielectric resonant element along the axis of the dielectric resonant element. The present invention also relates to a dielectric filter using the dielectric resonator.
2. Description of the Related Art
The structure of a conventional dielectric resonator 110 will be described with reference to FIG. 7. In FIG. 7, a frame 111 and a cross-shaped dielectric resonant element 112, which is formed by two column-shaped dielectics orthogonaly integrated with each other, are generally formed in one piece of dielectric ceramic. Concave portions 114 are formed from the external surface of the frame 111 in the directions of each axis of the crossing portions of the dielectric resonant element 112. These concave portions 114 are formed with the intent of miniaturizing the dielectric resonator and multiplexing the mode. A conductor 113 covers the overall external surface of the frame 111 including inside the concave portions 114. The conductor 113 may be formed, for example, by applying silver paste followed by burning.
Next, a dielectric filter 120 utilizing the dielectric resonator will be described with reference to FIG. 8. In FIG. 8, a metal panel 121 is fixed to the dielectric resonator so as to cover the opening of the dielectric resonator, and a metal case 122 accommodates the dielectric resonator. The metal panel 121 is fixed to the dielectric resonator 110 by soldering. An external connector 123 for input-output of signals is formed on the metal panel 121, and a loop 124 for connecting is attached to the external connector 123. The metal case 122 plays the role of the functions of fixing of the external connector 123 and the dielectric resonator 110 and protecting the dielectric resonator from an external impact. In order to fix the dielectric resonator 110, spring forces exerted by protruding portions 125 which are formed on the metal case 122 extending inwardly are utilized. The dielectric resonator 110 is fixed at a plurality of points where these protruding portions are located.
The dielectric resonator generates heat when fed a current from the outside. If the dielectric resonator cannot dissipate the heat, the temperature of the dielectric resonator increases with the generation of the heat causing characteristics of the dielectric resonator to deteriorate because of reduction of Qo (Q at no-load) and the like. The heat of the aforementioned concave portions on the dielectric resonator, where there is a large amount of heat generated, is difficult to be dissipated by convection because of the inwardly concave structure. Accordingly, the amount of heat at the concave portions may increase. The problem of the heat dissipating from the concave portions has been particularly acute.
When the dielectric resonator is fixed to a metal panel and is encased in a case to be used at a base station, etc. as a dielectric filter, the heat-dissipating problem becomes more serious because of increased current from the outside.
The heat-dissipating processes of the aforementioned dielectric filter are now described. The heat generated at the dielectric resonator is dissipated through three routes. First, there is a heat-conducting route from the dielectric resonator to the metal case through the metal panel. In this route, the heat is conducted through metals having a high thermal conductivity. However, since the dielectric resonator is fixed to the case by the protruding portions which extends inwardly from the case, as described above, there are only several contact points between the metal panel and the metal case and the contact area is so small as not to dissipate sufficient heat. Secondly, there is a convection route of air which is located in a clearance between the dielectric resonator and the case. Since the protruding portions are formed as to be recessed inwardly, it is difficult for heat to dissipate by convection through this route. Finally, there is a heat radiation route. However, there cannot be a sufficient amount of heat radiated to prevent the temperature of the dielectric resonator from rising.
The temperature in the dielectric filter is raised because there is insufficient heat dissipation at the dielectric resonator especially at the concave portions as described above. Therefore, the temperature of the solder used to connect the dielectric resonator to the metal panel is also raised. Because solder generally has a low melting point, the solder may partially melt by the rise in temperature. This results in displacing of the metal panel and variations in a resonating cavity. Therefore, long-life high reliability of the dielectric filter cannot be ensured.